First, consider what is necessary to generate a voluntary movement. For example, to make a reaching movement with your arm, you must first identify the target (or goal) and locate it in external space. Next, a limb trajectory must be determined on the basis of an internal representation of your arm and, in particular, your hand in relation to the target. Finally, a set of forces necessary to generate the desired trajectory must be computed. This process is often thought of as a series of transformations between coordinate systems. For example, the location of a visually identified target is measured in a retinotopic space, but its location is perceived in an external or world space (i.e., the position of a nonmoving target is perceived as stable, even when the eye, and thus the target’s image on the retina, changes). Next, calculation of a trajectory would involve a body-or hand-centered system, and finally, forces must ultimately be computed in a muscle-based reference frame.
These steps form a linear sequence, and traditionally it was thought that a hierarchy of motor areas carried out the successive steps. For example, the target of the movement was thought to be identified by pooling of sensory information in the posterior parietal cerebral cortex (Fig.
9.14A ). This information would then be transmitted to the supplementary motor and premotor areas, where a motor plan would be developed and then forwarded to the primary motor cortex, whose activity would be related to the final execution stage (e.g., generation of appropriate force levels). The motor cortex would then transmit commands, via the descending pathways discussed earlier, to the spinal cord and brainstem motor nuclei.
Although there is significant evidence in support of this hierarchical view of the generation of voluntary movement by the cortical motor system, more recent results have suggested a different conception: namely, that the various motor areas should be thought of as forming a parallel distributed network rather than a strict hierarchy (see
Fig.
9.14C ). For example, each cortical motor area makes its own significant contribution to the descending motor pathways; the primary motor cortex contributes only approximately half the fibers in the corticospinal tract that arise from the frontal lobe. Moreover, the various motor areas are all bidirectionally connected to each other, and results of the single-unit recording studies described later suggest that each of the areas plays a role in several of the stages of planning and executing a movement. This debate forms one of the themes of the following discussion because in its various guises, the distributed network versus hierarchical organization debate has been ongoing for decades and will probably continue for some time.
The motor areas in the cerebral cortex were originally defined on the basis of experiments in which electrical stimuli applied to the cortex evoked discrete, contralateral movement. Movement, however, can also be evoked when other cortical areas are stimulated more intensely. Thus motor areas are defined as those from which movement can be evoked by the lowest stimulus intensity. On the basis of these stimulation studies, the effects produced by lesions, results of anatomical experiments, electrophysiological recordings, and modern imaging studies in humans, many “motor” areas of the cerebral cortex have been recognized (see
Fig. 9.14 ), including the primary motor cortex in the precentral gyrus, the premotor area just rostral to the primary motor cortex, the supplementary motor cortex on the medial aspect of the hemisphere, and three cingulate motor areas located on the walls of the cingulate sulcus in the frontal lobe. There are also cortical regions in other lobes whose activity is related specifically to eye movement (see the section
Somatotopic Organization of Cortical Motor Areas
The primary motor cortex (or just motor cortex) can be defined as the region of cortex from which movements are elicited with the least amount of electrical stimulation. It is essentially congruent with the Brodmann cytoarchitectonic area 4 (see